Optical tunable filters are used in optical communications systems for optical performance monitoring (OPM). Other applications for tunable optical filters, inter alia, are for optical noise filtering, noise suppression, and wavelength division multiplexing.
For the purpose of describing the invention, the focus of the description below will be on tunable optical filters used in OPM systems, and OPM systems for wavelength division multiplexed (WDM) systems. It will be understood that the invention is not so limited.
In WDM systems, the basic design assumes wavelength stability. However, a variety of dynamic changes occur due to temperature changes, component aging, electrical power variations, etc. For optimum system performance it is necessary to monitor these changes and adjust system parameters to account for them. To accomplish this, optical channel monitors (OCMs), also known as optical performance monitors (OPMs), may be used to measure critical information data for the various channels in the WDM system. OPMs may monitor signal dynamics, determine system functionality, identify performance change, etc. In each case they typically provide feedback for controlling network elements to optimize operational performance. More specifically, these tunable optical filters scan the C−, L− and/or C+L-band wavelength range and precisely measure channel wavelength, power, and optical signal-to-noise ratio (OSNR).
However, components in the optical monitors also suffer from dynamic changes, changes similar to those affecting the WDM system components being monitored. These changes may track those of the system being monitored. Consequently, absolute wavelength drift in the WDM system components may be masked.
Accordingly it is desirable to periodically calibrate the tunable optical filters in the OPMs using a fixed reference wavelength source.
Fixed reference wavelength sources typically used for this purpose are multiple single wavelength reference lasers, or tunable reference lasers. The tunable optical filter being analyzed scans the reference wavelength source(s) and the output optical power measured. Results reveal drift or other errors in the tunable optical filter calibration.
Measuring for errors at a single wavelength using, for example, a single fixed wavelength laser source, will show whether a dynamic change in the wavelength response of the tunable optical filter has occurred, and the sign and magnitude of that change. However, that approach shows only changes for the single wavelength. A more thorough analysis of a tunable optical filter requires multiple wavelength measurements. Measurements at two wavelengths provide the additional data of the slope of the wavelength shift. In many cases, it is valid to assume that the change is monotonic and linear. Thus measuring response of the filter at two wavelengths is sufficient for an effective system monitor. Measurements at more than two wavelengths show non-linear changes. For some applications that is desirable.
A wavelength reference source that allows measurements at multiple wavelengths is a tunable laser. However, tunable lasers are expensive. It would be desirable to have a method for achieving multiple wavelength measurements with a simpler and less expensive wavelength reference source.